1. Field of the Invention
This invention relates to encapsulated pigment for use in ink-jet ink formulations, improved ink-jet ink formulations using the encapsulated pigment, and methods for producing same.
2. Description of Related Art
Ink-jet printing is a conventional technique by which printing is normally accomplished without contact between the printing apparatus and the medium on which the desired print characters are deposited. Such printing is accomplished by ejecting ink from an ink-jet printhead of the printing apparatus via numerous methods that employ, for example, pressurized nozzles, electrostatic fields, piezo-electric elements and/or heaters for vapor-phase bubble formation.
Ink compositions used in ink-jet printing typically employ water, colorants and low molecular weight water-soluble or water-miscible solvents. The colorants are generally selected from dyes and pigments. Dye-based inks, while in general superior in color properties, have several disadvantages as compared to pigment-based inks. For example, dyes are usually water-soluble and remain so after drying on the print medium, and thus are easily re-dissolved by water spills. In addition, dyes typically exhibit relatively poor light stability relative to pigments and are known to fade even under normal office lighting. Thus, dye-based inks are often unsuitable for use in applications requiring water resistance and light stability. Consequently, pigments are generally preferred colorants for ink-jet ink formulations.
Unfortunately, the use of pigments in ink-jet ink formulations is not problem free. For example, pigments are typically water insoluble and exist as discrete particles in the final ink formulation and have a tendency to aggregate. The insoluble pigment particles are typically stabilized as a dispersion by use of a polymer dispersant component having a hydrophobic segment and a hydrophilic segment. Generally speaking, most pigment inks stabilized with polymer dispersants in aqueous media are based on an electosteric stabilizing mechanism in which the hydrophobic segment of the dispersant acts as an anchor adsorbed onto the surface of the pigment particle through acid-base, electron donor/acceptor, Van der Waals forces, or physical absorption. In such a system, the hydrophilic segment of the dispersant is extended into the aqueous medium to keep the dispersant dispersed and to set up an electosteric layer to prevent aggregation of the pigment particles.
Many interrelated and competing forces are present in these complex pigmented ink formulations, consequently, it is extremely difficult to formulate a polymer dispersant system for pigmented ink-jet inks that simultaneously provide improved stability, water-fastness, light-fastness, smear resistance, highlighter resistance, minimized nozzle clogging and print density.
It is desirable to produce a final ink formulation having as many of these properties as possible, especially smear resistance and highlighter resistance. Conventional methods for producing pigment-based ink-jet inks using conventional polymer dispersants fall short since they typically have inferior film-forming properties. Pigment-to-dispersant ratio plays an important role in the film-forming property versus optical density. For example, a higher level of dispersant will typically improve smear resistance but at the expense of optical density. One conventional solution to this problem is to use a binder to improve film-forming properties, such as those taught in U.S. Pat. No. 6,063,834 which is incorporated herein by reference. Binders are typically very sensitive to heat and thus are not compatible with thermal ink-jet print heads. Therefore, the amount of binder that can be added to the ink is limited and thus, the ink does not produce high quality print.
Therefore, there still remains a need in the art for ink-jet ink formulations and manufacturing techniques that leads to print having improved properties.